1. Technical Field
The present disclosure relates generally to radio frequency (RF) communications switch devices, and in particular, complementary metal oxide semiconductor (CMOS) differential antenna transmit-receive switches with power combining circuitry for orthogonal frequency-division multiplexing (OFDM) systems.
2. Related Art
Generally, wireless communications involve a radio frequency (RF) carrier signal that is variously modulated to represent data, and the modulation, transmission, receipt, and demodulation of the signal conform to a set of standards for coordination of the same. Many different mobile communication technologies or air interfaces exist, including GSM (Global System for Mobile Communications), EDGE (Enhanced Data rates for GSM Evolution), and UMTS (Universal Mobile Telecommunications System) W-CDMA (Wideband Code Division Multiple Access). More recently, 4G (fourth generation) technologies such as LTE (Long Term Evolution), which is based on the earlier GSM and UMTS standards, are being deployed. Besides these mobile communications modalities, there are local area data networking modalities such as Wireless LAN (WLAN)/WiFi, WiMax, and so forth.
A fundamental component of any wireless communications system is the transceiver, that is, the combined transmitter and receiver circuitry. The transceiver encodes the data to a baseband signal and modulates it with an RF carrier signal. Upon receipt, the transceiver down-converts the RF signal, demodulates the baseband signal, and decodes the data represented by the baseband signal. An antenna connected to the transmitter converts the electrical signals to electromagnetic waves, and an antenna connected to the receiver converts the electromagnetic waves back to electrical signals.
Depending on the particulars of the communications modality, single or multiple antennas may be utilized. The output of the transmitter is connected to a power amplifier, which amplifies the RF signals prior to transmission via the antenna. The receiver is connected to the output of a low noise amplifier, the input of which is connected to the antenna and receives inbound RF signals. A transmit/receive switch selectively interconnects the antenna to the output of the power amplifier during transmission, and to the input of the low noise amplifier during reception. Thus, the power amplifier, the low noise amplifier, and the antenna switch serves as key building blocks in RF transceiver circuitry. These components may be referred to as a front end circuit.
Conventionally, in order to lower manufacturing costs and allow full integration of a complete RF System-on-Chip (SoC), a complementary metal oxide semiconductor (CMOS) technology is utilized for the power amplifier and the antenna switch circuitry. SoC devices with integrated front end circuits intended for mobile communications applications require both a high sensitivity receiver, a power amplifier with a low error vector magnitude (EVM) floor, and a local oscillator, all on a single semiconductor die. Local oscillator pulling and substrate noise coupling render differential amplifiers a robust choice, and small form factor integrated circuits suitable for mobile applications are possible with differential circuits that incorporate coupled inductors.
Existing transmit/receive or antenna switches are implemented in CMOS technology with series-shunt NMOS (n-type metal oxide semiconductor) transistors in the transmit and receive paths to the antenna. The series switch either passes or blocks the respective signal path. The shunt switch grounds the undesired signal and isolates the downstream component, but there is an associated increase in insertion loss. Another approach involves utilizing a resonant inductor with the switch in the off state to achieve a higher impedance as seen by the antenna.
However, there are several inherent limitations associated with CMOS switch devices. One of the most significant is low breakdown voltage, which is exacerbated by the trend towards SoC devices utilizing standard nanometer processes, together with increases in output power. Additionally, balun and transformers with ultra-high coupling factors have been difficult to produce using standard CMOS processes. It may be possible to utilize dual or multi-integrated circuit device MIMO (multiple input, multiple output) front end circuit, but such an implementation results in greatly increased costs.
Accordingly, there is a need in the art for a differential front end architecture that can reduce voltage stress across a CMOS switch. There is a need for such voltage stress to be reduced by half for the same transmitted power at the antenna.